Cancers are characterized by unregulated cell growth, tissue invasion, and metastasis. A neoplasm is benign when it grows in an unregulated fashion without tissue invasion. The presence of both features is characteristic of malignant neoplasms. Cancers are named based on their origin: those derived from epithelial tissue are called carcinomas, those derived from mesenchymal tissues are sarcomas, and those derived from hematopoietic tissue are leukemias or lymphomas.
Cancers nearly always arise as a consequence of genetic alterations. Choriocarcinoma may be an exception to this rule in that experimental insertion of a choriocarcinoma cell into an animal blastocyst can result in the neoplastic cell giving rise to normal body structures under the inductive influence of the developing embryo. Such an occurrence would be unlikely in the setting of irreversible genetic damage.
Occasional cancers appear to be caused by an alteration in a dominant gene that drives uncontrolled cell proliferation. Examples include chronic myeloid leukemia (abl) and Burkitt's lymphoma (c-myc). The genes that can promote cell growth when altered are often called oncogenes. They were first identified as critical elements of viruses that cause animal tumors; later it was found that the viral genes had normal counterparts with important functions in the cell and had been captured and mutated by viruses as they passed from host to host.
However, the vast majority of human cancers are characterized by multiple genetic abnormalities, each of which contributes to the loss of control of cell proliferation and differentiation and the acquisition of capabilities, such as tissue invasion and angiogenesis. Many cancers go through recognizable steps of progressively more abnormal phenotypes: hyperplasia to adenoma to dysplasia to carcinoma in situ to invasive cancer (Table 25-1). These properties are not found in the normal adult cell from which the tumor is derived. Indeed, normal cells have a large number of safeguards against uncontrolled proliferation and invasion.
TABLE 25-1PHENOTYPIC CHARACTERISTICS OF MALIGNANT CELLS ||Download (.pdf) TABLE 25-1PHENOTYPIC CHARACTERISTICS OF MALIGNANT CELLS
|Deregulated cell proliferation: Loss of function of negative growth regulators (suppressor oncogenes, i.e., Rb, p53), and increased action of positive growth regulators (oncogenes, i.e., Ras, Myc). Leads to aberrant cell cycle control and includes loss of normal checkpoint responses. |
|Failure to differentiate: Arrest at a stage before terminal differentiation. May retain stem cell properties. (Frequently observed in leukemias due to transcriptional repression of developmental programs by the gene products of chromosomal translocations.) |
|Loss of normal apoptosis pathways: Inactivation of p53, increases in Bcl-2 family members. This defect enhances the survival of cells with oncogenic mutations and genetic instability and allows clonal expansion and diversification within the tumor without activation of physiologic cell death pathways. |
|Genetic instability: Defects in DNA repair pathways leading to either single or oligo-nucleotide mutations (as in microsatellite instability, MIN) or more commonly chromosomal instability (CIN) leading to aneuploidy. Caused by loss of function of p53, BRCA1/2, mismatch repair genes, DNA repair enzymes, and the spindle checkpoint. |
|Loss of replicative senescence: Normal cells stop dividing in vitro after 25–50 population doublings. Arrest is mediated by the Rb, p16INK4a, and p53 pathways. Further replication leads to telomere loss with crisis. Surviving cells often harbor gross chromosomal abnormalities. Relevance to human in vivo cancer remains uncertain. Many human cancers express telomerase. |
|Increased angiogenesis: Due to increased gene expression of proangiogenic factors (VEGF, FGF, IL-8) by tumor or stromal cells, or loss of negative regulators (endostatin, tumstatin, thrombospondin). |
|Invasion: Loss of cell-cell contacts (gap junctions, cadherins) and increased production of matrix metalloproteinases (MMPs). Often takes the form of epithelial-to-mesenchymal transition (EMT), with anchored epithelial cells becoming more like motile fibroblasts. |
|Metastasis: Spread of tumor cells to lymph nodes or distant tissue sites. Limited by the ability of tumor cells to survive in a foreign environment. |
|Evasion of the immune system: Downregulation of MHC class I and II molecules; induction of T cell tolerance; inhibition of normal dendritic cell and/or T cell function; antigenic loss variants and clonal heterogeneity; increase in regulatory T cells. |
In most organs, only primitive nonfunctional cells are capable of proliferating, and the cells lose the capacity to proliferate as they differentiate and acquire functional capability. The expansion of the primitive cells is linked to some functional need in the host through receptors that receive signals from the local environment or through hormonal influences delivered by the vascular supply. In the absence of such signals, the cells are at rest. We have a poor understanding of the signals that keep the primitive cells at rest. These signals, too, must be environmental, based on the observations that a regenerating liver stops growing when it has replaced the portion that has been surgically removed and regenerating bone marrow stops growing when the peripheral blood counts return to normal. Cancer cells clearly have lost responsiveness to such controls and do not recognize when they have overgrown the niche normally occupied by the organ from which they are derived. We know very little about this mechanism of growth regulation.
Normal cells have a number of control mechanisms that are targeted by specific genetic alterations in cancer. The progression of a cell through the cell division cycle is regulated at a number of checkpoints by a wide array of genes. In the first phase, G1, preparations are made to replicate the genetic material. The cell stops before entering the DNA synthesis phase or S phase to take inventory. Are we ready to replicate our DNA? Is the DNA repair machinery in place to fix any mutations that are detected? Are the DNA replicating enzymes available? Is there an adequate supply of nucleotides? Is there sufficient energy? The main brake on the process is the retinoblastoma protein, Rb. When the cell determines that it is prepared to move ahead, sequential activation of cyclin-dependent kinases (CDKs) results in the inactivation of the brake, Rb, by phosphorylation. Phosphorylated Rb releases the S-phase-regulating transcription factor, E2F/DP1, and genes required for S phase progression are expressed. If the cell determines that it is unready to move ahead with DNA replication, a number of inhibitors are capable of blocking the action of the CDKs, including p21Cip2/Waf1, p16Ink4a, and p27Kip1. Nearly every cancer has one or more genetic lesions in the G1 checkpoint that permits progression to S phase.
At the end of S phase, when the cell has exactly duplicated its DNA content, a second inventory is taken at the S checkpoint. Have all of the chromosomes been fully duplicated? Were any segments of DNA copied more than once? Do we have the right number of chromosomes and the right amount of DNA? If so, the cell proceeds to G2, in which the cell prepares for division by synthesizing mitotic spindle and other proteins needed to produce two daughter cells. When DNA damage is detected, the p53 pathway is normally activated. Called the guardian of the genome, p53 is a transcription factor that is normally present in the cell in very low levels. Its level is generally regulated through its rapid turnover. Normally, p53 is bound to mdm2, which transports p53 out of the nucleus for degradation in the proteosome. When damage is sensed, the ATM (ataxia-telangiectasia mutated) pathway is activated; ATM phosphorylates mdm2, which no longer binds to p53, and p53 then stops cell cycle progression and directs the synthesis of repair enzymes, or if the damage is too great, initiates apoptosis of the cell to prevent the propagation of a damaged cell (Fig. 25-1).
Induction of p53 by the DNA damage and oncogene checkpoints. In response to noxious stimuli, p53 and mdm2 are phosphorylated by the ataxia-telangiectasia mutated (ATM) and related ATR serine/threonine kinases, as well as the immediate downstream checkpoint kinases, Chk1 and Chk2. This causes dissociation of p53 from mdm2, leading to increased p53 protein levels and transcription of genes leading to cell cycle arrest (p21Cip1/Waf1) or apoptosis (e.g., the proapoptotic Bcl-2 family members Noxa and Puma). Inducers of p53 include hypoxemia, DNA damage (caused by ultraviolet radiation, gamma irradiation, or chemotherapy), ribonucleotide depletion, and telomere shortening. A second mechanism of p53 induction is activated by oncogenes such as Myc, which promote aberrant G1/S transition. This pathway is regulated by a second product of the Ink4a locus, p14ARF (p19 in mice), which is encoded by an alternative reading frame of the same stretch of DNA that codes for p16Ink4a. Levels of ARF are upregulated by Myc and E2F, and ARF binds to mdm2 and rescues p53 from its inhibitory effect. This oncogene checkpoint leads to the death or senescence (an irreversible arrest in G1 of the cell cycle) of renegade cells that attempt to enter S phase without appropriate physiologic signals. Senescent cells have been identified in patients whose premalignant lesions harbor activated oncogenes, for instance, dysplastic nevi that encode an activated form of BRAF (see later discussion), demonstrating that induction of senescence is a protective mechanism that operates in humans to prevent the outgrowth of neoplastic cells.
A second method of activating p53 involves the induction by oncogenes of p14ARF (p19 in mice). ARF competes with p53 for binding to mdm2, allowing p53 to escape the effects of mdm2 and accumulate in the cell. Then p53 stops cell cycle progression by activating CDK inhibitors such as p21 and/or initiating the apoptosis pathway. Mutations in the gene for p53 on chromosome 17p are found in more than 50% of human cancers. Most commonly these mutations are acquired in the malignant tissue in one allele and the second allele is deleted, leaving the cell unprotected from DNA-damaging agents. Some environmental exposures produce signature mutations in p53; for example, aflatoxin exposure leads to mutation of arginine to serine at codon 249 and leads to hepatocellular carcinoma. In rare instances, p53 mutations are in the germ line (Li-Fraumeni syndrome) and produce a familial cancer syndrome. The absence of p53 leads to chromosome instability and the accumulation of DNA damage, including the acquisition of properties that give the abnormal cell a proliferative and survival advantage. Like Rb dysfunction, most cancers have mutations that disable the p53 pathway. Indeed, the importance of p53 and Rb in the development of cancer is underscored by the neoplastic transformation mechanism of human papillomavirus. This virus has two main oncogenes, E6 and E7. E6 acts to increase the rapid turnover of p53, and E7 acts to inhibit Rb function; inhibition of these two targets is sufficient to lead to neoplasia.
Another cell cycle checkpoint exists when the cell is undergoing division, the spindle checkpoint. The details of this checkpoint are still being discovered; however, it appears that if the spindle apparatus does not properly align the chromosomes for division, if the chromosome number is abnormal (i.e., > or <4n), if the centromeres are not properly paired with their duplicated partners, then the cell initiates a cell death pathway to prevent the production of aneuploid progeny. Abnormalities in the spindle checkpoint facilitate the development of aneuploidy. In some tumors, aneuploidy is a predominant genetic feature. In others, microsatellite instability is the primary genetic lesion. Microsatellite instability arises from defects in DNA mismatch repair genes. In general, tumors either have defects in chromosome number or microsatellite instability but not both. Defects that lead to cancer include abnormal cell cycle checkpoints, inadequate DNA repair, and failure to preserve genome integrity.
Efforts are underway to therapeutically restore the defects in cell cycle regulation that characterize cancer.
CANCER AS AN ORGAN THAT IGNORES ITS NICHE
The fundamental cellular defects that create a malignant neoplasm act at the cellular level. However, that is not the entire story. Cancers behave as organs that have lost their specialized function and stopped responding to signals that normally limit their growth. Human cancers usually become clinically detectable when a primary mass is at least 1 cm in diameter—such a mass consists of about 109 cells. More commonly, patients present with tumors that are 1010 cells or greater. A lethal tumor burden is about 1012 cells. If all tumor cells were dividing at the time of diagnosis, patients would reach a lethal tumor burden in a very short time. However, human tumors grow by Gompertzian kinetics—this means that not every daughter cell produced by a cell division is itself capable of dividing. The growth fraction of a tumor declines exponentially with time. The growth fraction of the first malignant cell is 100%, and by the time a patient presents for medical care, the growth fraction is 2–3% or less. This fraction is similar to the growth fraction of normal bone marrow and normal intestinal epithelium, the most highly proliferative normal tissues in the human body, a fact that may explain the dose-limiting toxicities of agents that target dividing cells.
The implication of these data is that the tumor is slowing its own growth over time. How does it do this? The tumor cells have multiple genetic lesions that tend to promote proliferation, yet by the time the tumor is clinically detectable, its capacity for proliferation has declined. We need to better understand how a tumor stops its own growth. A number of factors can contribute to the failure of tumor cells to proliferate in vivo. Some cells are hypoxemic and have an inadequate supply of nutrients and energy. Some have sustained too much genetic damage to complete the cell cycle and have lost the capacity to undergo apoptosis. However, an important subset is not actively dividing but retains the capacity to divide and starts dividing again when the tumor mass is reduced by treatments. Just as the bone marrow increases its rate of proliferation in response to bone marrow–damaging agents, so too does the tumor seem to sense when the tumor cell numbers have been reduced and responds by increasing its growth rate. However, the marrow stops growing when it has reached its production goals. Tumors do not.
It is not in the long-term interest of a cancer to kill its host. It errs when it overshoots the limits imposed by the organ niche it occupies. Additional tumor cell vulnerabilities are likely to be detected when we learn more about how normal cells respond to "stop" signals from their environment and how tumor cells fail to heed such signals.
IS IN VITRO SENESCENCE RELEVANT TO CARCINOGENESIS?
When normal cells are placed in culture in vitro, most are not capable of sustained growth. Fibroblasts are an exception to this rule. When they are cultured, fibroblasts may divide 30–50 times, and then they undergo what has been termed a "crisis" during which the majority of cells stop dividing (usually due to an increase in p21 expression, a CDK inhibitor), many die, and a small fraction emerge that have acquired genetic changes that permit their uncontrolled growth. The cessation of growth of normal cells in culture has been termed "senescence," and whether this phenomenon is relevant to any physiologic event in vivo is debated.
Among the cellular changes during in vitro propagation is telomere shortening. DNA polymerase is unable to replicate the tips of chromosomes, resulting in the loss of DNA at the specialized ends of chromosomes (called telomeres) with each replication cycle. At birth, human telomeres are 15- to 20-kb pairs long and are composed of tandem repeats of a six-nucleotide sequence (TTAGGG) that associates with specialized telomere-binding proteins to form a T-loop structure that protects the ends of chromosomes from being mistakenly recognized as damaged. The loss of telomeric repeats with each cell division cycle causes gradual telomere shortening, leading to growth arrest (called senescence) when one or more critically short telomeres trigger a p53-regulated DNA-damage checkpoint response. Cells can bypass this growth arrest if pRb and p53 are nonfunctional, but cell death ensues when the unprotected ends of chromosomes lead to chromosome fusions or other catastrophic DNA rearrangements. The ability to bypass telomere-based growth limitations is thought to be a critical step in the evolution of most malignancies. This occurs by the reactivation of telomerase expression in cancer cells. Telomerase is an enzyme that adds TTAGGG repeats onto the 3′ ends of chromosomes. It contains a catalytic subunit with reverse transcriptase activity (hTERT) and an RNA component that provides the template for telomere extension. Most normal somatic cells do not express sufficient telomerase to prevent telomere attrition with each cell division. Exceptions include stem cells (such as those found in hematopoietic tissues, gut and skin epithelium, and germ cells) that require extensive cell division to maintain tissue homeostasis. More than 90% of human cancers express high levels of telomerase that prevent telomere shortening to critical levels and allow indefinite cell proliferation. In vitro experiments indicate that inhibition of telomerase activity leads to tumor cell apoptosis. Major efforts are underway to develop methods to inhibit telomerase activity in cancer cells. The reverse transcriptase activity of telomerase is a prime target for small-molecule pharmaceuticals. In addition, the protein component of telomerase (hTERT) may act as a tumor-associated antigen and be targeted by vaccine approaches.
All of the known functions of telomerase relate to cell division. Thus, it is unclear how short telomeres interfere with the differentiated functions of normal cells. Nevertheless, a major growth industry in medical research has been discovering an association between short telomeres and human diseases ranging from diabetes and coronary artery disease to Alzheimer's disease. The picture is further complicated by the fact that rare genetic defects in the telomerase enzyme seem to cause pulmonary fibrosis but not hematopoietic failure or defects in nutrient absorption in the gut, the sites that might be presumed to be most sensitive to defective cell proliferation. Much remains to be learned about how telomere shortening and telomere maintenance is related to human illness in general and cancer in particular.
SIGNAL TRANSDUCTION PATHWAYS IN CANCER CELLS
Signals that affect cell behavior come from adjacent cells, the stroma in which the cells are located, hormonal signals that originate remotely, and the cells themselves (autocrine signaling). These signals generally exert their influence on the receiving cell through activation of signal transduction pathways that have as their end result the induction of activated transcription factors that mediate a change in cell behavior or function or the acquisition of effector machinery to accomplish a new task. Although signal transduction pathways can lead to a wide variety of outcomes, many such pathways rely on cascades of signals that sequentially activate different proteins or glycoproteins and lipids or glycolipids, and the activation steps often involve the addition or removal of one or more phosphate groups on a downstream target. Other chemical changes can result from signal transduction pathways, but phosphorylation and dephosphorylation play a major role. The protein kinases are generally of two distinct classes; one class acts on tyrosine residues, and the other acts on serine/threonine residues. The tyrosine kinases often play critical roles in signal transduction pathways; they may be receptor tyrosine kinases or may be linked to other cell-surface receptors through associated docking proteins (Fig. 25-2).
Therapeutic targeting of signal transduction pathways in cancer cells. Three major signal transduction pathways are activated by receptor tyrosine kinases (RTK). 1. The protooncogene Ras is activated by the Grb2/mSOS guanine nucleotide exchange factor, which induces an association with Raf and activation of downstream kinases (MEK and ERK1/2). 2. Activated PI3K phosphorylates the membrane lipid PIP2 to generate PIP3, which acts as a membrane-docking site for a number of cellular proteins including the serine/threonine kinases PDK1 and Akt. PDK1 has numerous cellular targets, including Akt and mTOR. Akt phosphorylates target proteins that promote resistance to apoptosis and enhance cell cycle progression, while mTOR and its target p70S6K upregulate protein synthesis to potentiate cell growth. 3. Activation of PLC-γ leads the formation of diacylglycerol (DAG) and increased intracellular calcium, with activation of multiple isoforms of PKC and other enzymes regulated by the calcium/calmodulin system. Other important signaling pathways involve non-RTKs that are activated by cytokine or integrin receptors. Janus kinases (JAK) phosphorylate STAT (signal transducer and activator of transcription) transcription factors, which translocate to the nucleus and activate target genes. Integrin receptors mediate cellular interactions with the extracellular matrix (ECM), inducing activation of FAK (focal adhesion kinase) and c-Src, which activate multiple downstream pathways, including modulation of the cell cytoskeleton. Many activated kinases and transcription factors migrate into the nucleus, where they regulate gene transcription, thus completing the path from extracellular signals, such as growth factors, to a change in cell phenotype, such as induction of differentiation or cell proliferation. The nuclear targets of these processes include transcription factors (e.g., Myc, AP-1, and serum response factor) and the cell cycle machinery (CDKs and cyclins). Inhibitors of many of these pathways have been developed for the treatment of human cancers. Examples of inhibitors that are currently being evaluated in clinical trials are shown in purple type.
Normally, tyrosine kinase activity is short-lived and reversed by protein tyrosine phosphatases (PTPs). However, in many human cancers, tyrosine kinases or components of their downstream pathways are activated by mutation, gene amplification, or chromosomal translocations. Because these pathways regulate proliferation, survival, migration, and angiogenesis, they have been identified as important targets for cancer therapeutics.
Inhibition of kinase activity is effective in the treatment of a number of neoplasms. Lung cancers with mutations in the epidermal growth factor receptor are highly responsive to erlotinib and gefitinib (Table 25-2). Lung cancers with activation of the anaplastic lymphoma kinase (ALK) respond to crizotinib, an ALK inhibitor. A BRAF inhibitor is highly effective in melanomas and thyroid cancers in which BRAF is overexpressed. Janus kinase inhibitors are active in myeloproliferative syndromes in which JAK2 activation is a pathogenetic event. Imatinib is an effective agent in tumors that overexpress c-Abl (such as chronic myeloid leukemia), c-Kit (gastrointestinal stromal cell tumors), or platelet-derived growth factor receptor (PDGFR; chronic myelomonocytic leukemia); second-generation congeners, dasatinib, and nilotinib are even more effective. Sorafenib and sunitinib, agents that inhibit a large number of kinases, are being widely tested and have shown promising antitumor activity in renal cell cancer and hepatocellular carcinoma. Inhibitors of the mammalian target of rapamycin (mTOR) such as temsirolimus are also active in renal cell cancer. The list of active agents and treatment indications is growing rapidly. These new agents have ushered in a new era of personalized therapy. It is becoming more routine for resected tumors to be assessed for specific molecular changes that predict response and to have clinical decision-making guided by those results.
TABLE 25-2SOME FOOD AND DRUG ADMINSTRATION–APPROVED MOLECULARLY TARGETED AGENTS FOR THE TREATMENT OF CANCER ||Download (.pdf) TABLE 25-2SOME FOOD AND DRUG ADMINSTRATION–APPROVED MOLECULARLY TARGETED AGENTS FOR THE TREATMENT OF CANCER
|DRUG ||MOLECULAR TARGET ||DISEASE ||MECHANISM OF ACTION |
|All-trans retinoic acid (ATRA) ||PML-RARα oncogene ||Acute promyelocytic leukemia M3 AML; t(15;17) ||Inhibits transcriptional repression by PML-RARα |
|Bcr-Abl, c-Abl, c-Kit, PDGFR-α/β ||Chronic myeloid leukemia; GIST ||Blocks ATP binding to tyrosine kinase active site |
|Sunitinib (Sutent) ||c-Kit, VEGFR-2, PDGFR-β, Flt-3 ||GIST; renal cell cancer ||Inhibits activated c-Kit and PDGFR in GIST; inhibits VEGFR in RCC |
|Sorafenib (Nexavar) ||RAF, VEGFR-2, PDGFR-α/β, Flt-3, c-Kit ||RCC; hepatocellular carcinoma ||Targets VEGFR pathways in RCC. Possible activity against BRAF in melanoma, colon cancer, and others |
|Erlotinib (Tarceva) ||EGFR ||Non–small cell lung cancer; pancreatic cancer ||Competitive inhibitor of the ATP-binding site of the EGFR |
|Gefitinib (Iressa) ||EGFR ||Non–small cell lung cancer ||Inhibitor of EGFR tyrosine kinase |
|Bortezomib (Velcade) ||Proteasome ||Multiple myeloma ||Inhibits proteolytic degradation of multiple cellular proteins |
|Monoclonal Antibodies |
|Trastuzumab (Herceptin) ||HER2/neu (ERBB2) ||Breast cancer ||Binds HER2 on tumor cell surface and induces receptor internalization |
|Cetuximab (Erbitux) ||EGFR ||Colon cancer, squamous cell carcinoma of the head and neck ||Binds extracellular domain of EGFR and blocks binding of EGF and TGF-α; induces receptor internalization. Potentiates the efficacy of chemotherapy and radiotherapy |
|Panitumumab (Vectibix) ||EGFR ||Colon cancer ||Like cetuximab; likely to be very similar in clinical activity |
|Rituximab (Rituxan) ||CD20 ||B cell lymphomas and leukemias that express CD20 ||Multiple potential mechanisms, including direct induction of tumor cell apoptosis and immune mechanisms |
|Alemtuzumab (Campath) ||CD52 ||Chronic lymphocytic leukemia and CD52-expressing lymphoid tumors ||Immune mechanisms |
|Bevacizumab (Avastin) ||VEGF ||Colon, lung, breast cancers; data pending in other tumors ||Inhibits angiogenesis by high-affinity binding to VEGF |
However, it must be acknowledged that none of these therapies is curative in any malignancy. The reasons for the failure to cure are not all defined. However, at least some causes of resistance are known. In some tumors, resistance to kinase inhibitors is related to an acquired mutation in the target kinase that inhibits drug binding. Many of these kinase inhibitors act as competitive inhibitors of the adenosine triphosphate (ATP)–binding pocket. ATP is the phosphate donor in these phosphorylation reactions. Mutation in the BCR-ABL kinase in the ATP–binding pocket (such as the tyrosine to isoleucine change at codon 315) can prevent imatinib binding. Other resistance mechanisms include altering other signal transduction pathways to bypass the inhibited pathway. Some kinase inhibitors are less specific for an oncogenic target than was hoped, and toxicities related to off-target kinase inhibition limit the use of the agent at a dose that would inhibit the cancer-relevant kinase. As resistance mechanisms become better defined, rational strategies to overcome resistance will emerge.
Another strategy to enhance the antitumor effects of targeted agents is to use them in rational combinations with each other and in empiric combinations with chemotherapy agents that kill cells in ways distinct from targeted agents. For example, in the c-Kit overexpressing gastrointestinal stromal tumor (GIST), resistance to imatinib develops due to secondary mutations in c-Kit, and many of these tumors are susceptible to treatment with the multitargeted tyrosine kinase (TK) inhibitor sunitinib that has activity against c-Kit as well as the PDGF and vascular endothelial growth factor (VEGF) receptors. Sunitinib is approved by the U.S. Food and Drug Administration (FDA) for treatment of patients with imatinib-resistant GIST or who are intolerant of imatinib (Table 25-2). Interestingly, tumors with mutations in exon 11 of c-Kit's juxtamembrane region are particularly sensitive to imatinib, whereas those with exon 9 mutations (extracellular domain) respond better to sunitinib than imatinib. In the future, primary therapy for GIST may be determined by the specific molecular defect in c-Kit.
While targeted therapies have not yet resulted in cures when used alone, their use in the adjuvant setting and when combined with other effective treatments has substantially increased the fraction of patients cured. For example, the addition of rituximab, an anti-CD20 antibody, to combination chemotherapy in patients with diffuse large B-cell lymphoma improves cure rates by 15–20%. The addition of trastuzumab, antibody to HER2, to combination chemotherapy in the adjuvant treatment of HER2-positive breast cancer reduces relapse rates by 50%.
Targeted therapies are being developed for the ras/mitogen-activated protein (MAP) kinase pathways, the hedgehog pathway, various angiogenesis pathways, and phospholipid signaling pathways such as the phosphatidylinositol-3-kinase (PI3K) and phospholipase C-gamma pathways, which are involved in a large number of cellular processes that are important in cancer development and progression.
One of the strategies for new drug development is to take advantage of so-called oncogene addiction. This situation (Fig. 25-3) is created when a tumor cell develops an activating mutation in an oncogene that becomes a dominant pathway with reduced contributions from auxiliary pathways. This dependency on a single pathway creates a cell that is vulnerable to inhibitors of the oncogene pathway. For example, cells harboring mutations in BRAF are very sensitive to MEK inhibitors.
Oncogene addiction and synthetic lethality: keys to discovery of new anticancer drugs. A. Normal cells receive environmental signals that activate signaling pathways (pathways A, B, and C) that together promote G1 to S phase transition and passage through the cell cycle. Inhibition of one pathway (such as pathway A by a targeted inhibitor) has no significant effect due to redundancy provided by pathways B and C. In cancer cells, oncogenic mutations lead over time to dependency on the activated pathway, with loss of significant input from pathways B and C. The dependency or addiction of the cancer cell to pathway A makes it highly vulnerable to inhibitors that target components of this pathway. Clinically relevant examples include Bcr-Abl (CML), amplified HER2/neu (breast cancer), overexpressed or mutated EGF receptors (lung cancer), and mutated BRAF (melanoma). B. Genes are said to have a synthetic lethal relationship when mutation of either gene alone is tolerated by the cell but mutation of both genes leads to lethality. Thus, in the example, mutant gene a and gene b have a synthetic lethal relationship, implying that the loss of one gene makes the cell dependent on the function of the other gene. In cancer cells, loss of function of a tumor-suppressor gene (wild-type designated gene A; mutant designated gene a) may render the cancer cells dependent on an alternative pathway of which gene B is a component. As shown in the figure, if an inhibitor of gene B can be identified, this can cause death of the cancer cell, without harming normal cells (which maintain wild-type function for gene A). High-throughput screens can now be performed using isogenic cell line pairs in which one cell line has a defined defect in a tumor-suppressor pathway. Compounds can be identified that selectively kill the mutant cell line; targets of these compounds have a synthetic lethal relationship to the tumor-suppressor pathway and are potentially important targets for future therapeutics. Note that this approach allows discovery of drugs that indirectly target deleted tumor-suppressor genes and hence greatly expands the list of physiologically relevant cancer targets.
Many transcription factors are activated by phosphorylation, which can be prevented by tyrosine- or serine/threonine kinase inhibitors. The transcription factor NF-κB is a heterodimer composed of p65 and p50 subunits that associate with an inhibitor, IκB, in the cell cytoplasm. In response to growth factor or cytokine signaling, a multi-subunit kinase called IKK (IκB-kinase) phosphorylates IκB and directs its degradation by the ubiquitin/proteasome system. NF-κB, free of its inhibitor, translocates to the nucleus and activates target genes, many of which promote the survival of tumor cells. Novel drugs called proteasome inhibitors block the proteolysis of IκB, thereby preventing NF-κB activation. For unexplained reasons, this is selectively toxic to tumor cells. The antitumor effects of proteasome inhibitors are more complicated and involve the inhibition of the degradation of multiple cellular proteins. Proteasome inhibitors (bortezomib [Velcade]) have activity in patients with multiple myeloma, including partial and complete remissions. Inhibitors of IKK are also in development, with the hope of more selectively blocking the degradation of IκB, thus "locking" NF-κB in an inhibitory complex and rendering the cancer cell more susceptible to apoptosis-inducing agents.
Estrogen receptors (ERs) and androgen receptors, members of the steroid hormone family of nuclear receptors, are targets of inhibition by drugs used to treat breast and prostate cancers, respectively. Tamoxifen, a partial agonist and antagonist of ER function, can mediate tumor regression in metastatic breast cancer and can prevent disease recurrence in the adjuvant setting. Tamoxifen binds to the ER and modulates its transcriptional activity, inhibiting activity in the breast but promoting activity in bone and uterine epithelium. Selective estrogen receptor modulators (SERMs) have been developed with the hope of a more beneficial modulation of ER activity, i.e., antiestrogenic activity in the breast, uterus, and ovary, but estrogenic for bone, brain, and cardiovascular tissues. Aromatase inhibitors, which block the conversion of androgens to estrogens in breast and subcutaneous fat tissues, have demonstrated improved clinical efficacy compared with tamoxifen and are often used as first-line therapy in patients with ER-positive disease (Chap. 37).